Edit - Just so it's right at the top. This is what you need to know:
ok it actually seems to work quite well after a bit of experimenting in the adb shell.
You'll need root but with that try the following:
Choose root mode
run every command as root and
Make sure "su"/superuser has access granted for all requests
Then go into lightflow, notification settings and you should have a
"mixer" option. Enable that and you should then see options to set the
red, green and blue leds and also an option to set their brightness.
The brightness is has a 255 max value (0 being off) - (255 max brightness) and it should work.
The led won't flash but you can mix them together to get different colors. Technically you don't need to use the mixer but there's more control with it from what I remember.
Also worth noting, that you can enable control of the "Charging" LED which is green only. If you use that LED, it's slightly brighter than using the standard "green" LED.
Also, brightness levels for the Charge LED are variable between 1 and 40. Over 40, it's no brighter.
The RGB LEDs seem to be on or off. I haven't seen any difference between brightness 1 and brightness 20 (or any other higher number, I just use 20 because the /sys/devices/leds-qpnp-f0067200/leds/red/max_brightness file indicates 20 is the max for this LED - it's 40 for the Charge LED)